PART Introduction I Earth System Science The Concept of Systems Types of Systems System Behavior Feedbacks in Earth Systems Earth’s Environmental Systems The Geosphere: Earth’s Metallic Interior and Rocky Outer Shell The Pedosphere: Where Soils Form The Hydrosphere: Earth’s Distinguishing Characteristic The Atmosphere: Earth’s Envelope of Gases The Biosphere: Where Life Exists Earth’s Energy System States of Energy Sources of Energy Energy Budget of Earth Human Consumption of Energy Human Population and Earth System Boundaries Human Population Growth Resources and Sustainable Development Pollution, Wastes, and Environmental Impact Natural Disasters, Hazards, and Risks The Anthropocene and Planetary Boundaries Dynamic Earth Systems CHAPTER 1

ust south of the equator in Tanzania looms the tall- est mountain in Africa, Mount Kilimanjaro, a mas- Jsive stratovolcano formed by hundreds of thousands of years’ worth of ash and lava flows piled one atop the other during volcanic eruptions. The peak stands among a chain of volcanoes along a great rift in Earth’s crust, the East African Rift valley, where the African tectonic plate is splitting apart. Mount Kilimanjaro rises so high (19,039 ft, or 5803 m) that its lower slopes are shrouded in tropical rain forest while its upper peaks are dry and bitter cold, masked with white glaciers and snow. The volcano is familiar to many as a literary icon from Ernest Hemingway’s story “The Snows of Kilimanjaro,” in which he wrote: “… and there, ahead ... as wide as all the world, great, high, and unbelievably white in the sun, was the square top of Kilimanjaro.” Today, Mount Kilimanjaro’s “unbelievably white” glaciers are nearly gone. Ascertaining the cause of their demise is a central concern of an international dialogue about the roles of scientific evidence and investigation in determining how human activities such as fossil fuel use and land cover change contribute to global warm- ing, glacier and sea ice melting, permafrost thawing, and sea level rise, as well as a host of other, related effects. The causes and effects of global change are complex and marked by numerous feedback processes, such as when the dark rocks and sediments exposed by shrinking glaciers lead to the greater absorption of incoming solar radiation that triggers atmospheric warming and still more melting. Despite the complexity of global change, understanding its processes, causes, and effects is crucial to our well-being as a species. In this book, we explore the Earth system’s myriad processes and feedbacks and consider the discoveries of scientists, as well as their ways of thinking, so that we can better evaluate the most pressing environmental concerns faced by humanity. To understand Earth’s interrelated processes and their impacts on humans, and to engage in ecological and physical restoration,

Mount Kilimanjaro’s glaciers can be seen from afar due to the stratovolcano’s great height within the East African Rift valley. (Fuse/Thinkstock)

3 4 PART I Introduction

we need to examine the smaller systems that interact in 1912 was gone by 2011.1 Most of the remaining gla- within the whole planetary system. In this chapter we: cial ice forms stagnant, isolated remnants. In 2000, scientists drilled six ice cores in these rem- ✔ Discuss the integrative field of Earth system nants, and from the cores they were able to document science. nearly 12,000 years of glacial history at the summit.2 ✔ Examine the concept of systems and their They discovered that glacial ice had persisted in the components. equatorial sun without periods of prolonged melting even through centuries-long droughts. The scientists ✔ Identify the forces that drive Earth’s processes and noted that the cause of recent glacial shrinking was examine how feedback mechanisms either amplify therefore likely to be global warming due to human or regulate them. activity. They added that mid- to low-latitude glaciers ✔ Consider the field of environmental geology and at high altitudes are shrinking rapidly worldwide, from the scope of this book. the Andes and Cascades in the western hemisphere to the Swiss Alps in Europe and the Himalayas in the eastern hemisphere, so the cause of their recent demise Earth System Science is likely to be global rather than local or regional. The scientists also suggested that the “snows of Kiliman- Large ice masses at Mount Kilimanjaro and else- jaro” might be gone as early as 2015. where existed during the last full glacial advance on Garnering international attention, this tentative Earth, which spanned the period from about 50,000 to prediction was cited in 2006 in a well-known docu- 16,000 years ago. Modern glaciers worldwide are rel- mentary, An Inconvenient Truth, which called for ics from that time, shrunken in size in response to the international action to stave off global warming. By relatively stable warm climatic conditions of the past 2010, however, it looked as if the glaciers on Mount approximately 11,000 years. Since the early 20th centu- Kilimanjaro—though further shrunken—might last a ry, however, Mount Kilimanjaro’s glaciers have withered few more decades. Furthermore, another group of rapidly, shrinking laterally and thinning (Figure 1-1). scientists reported that deforestation and forest fires Reconstructions from historic aerial photos and satellite at lower elevations in the region might have affected images reveal that 85 percent of the ice cover that existed regional weather patterns and diminished moisture at

314000 316000 318000 320000 UTM 37S 37°20'E 37°21'E 37°22'E 37°23'E WGS84 3°3'S 3°3'S 9662000 9662000 3°4'S 3°4'S

9660000 Figure 1-1 (a) Glaciers at the summit and side 9660000 slopes(b) of Mount Kilimanjaro are retreating rapidly, with about 85% of ice volume lost between 1912 3°5'S 3°5'S and 2012. (b) Scientific investigations use data from meteorological stations and snowpits, as well as other types of evidence, in order to reconstruct the history of glaciation and determine the causes of current glacial 9658000 9658000 shrinkage. (a: Cullen, N. J., Sirguey, P., Mölg, T., Kaser, G., Winkler, M., and Fitzsimons, S. J. 2013. “A Century of Ice 3°6'S 3°6'S 37°20'E 37°21'E 37°22'E 37°23'E Retreat on Kilimanjaro: The Mapping Reloaded.” The 314000 316000 318000 320000 Cryosphere 7: 419–431, © 2013 the authors; b: courtesy of Douglas Hardy, University of Massachusetts)

(a) CHAPTER 1 Dynamic Earth Systems 5

Figure 1-2 Earth’s 10,000 km environmental systems include Atmosphere the geosphere, pedosphere, hydrosphere, biosphere, and atmosphere. On continents, 12 km the interface of all five of these 10 km systems is the pedosphere. The top of the pedosphere, where all Earth’s systems interact, is Hydrosphere a relatively organic rich layer of Biosphere 5 km weathered rock called soil. Below the soil, weathered rock grades downward to the unweathered rock of the solid geosphere, Pedosphere 0 km forming an uneven layer up to 200 m thick.

–5 km Soil Lithosphere ( 2 m) Pedosphere (up to 200 m) Geosphere

Lithosphere

the volcano’s summit.3 Based on these findings, skeptics same function as a dam that holds water in a valley, of global warming were quick to claim that the first set and if glaciers are gone that resource will have vanished, of scientific studies could not verify that global warm- threatening the livelihoods of billions of people. Short of ing was the primary cause of glacial shrinking. If global a major change in climate toward cooler conditions, that warming were not the cause of shrinking glaciers, the loss of water resources will be permanent with respect to skeptics argued, it might be imprudent to make poten- the timescale of human interest. tially costly changes in long-established habits, such as The complexity of Earth system processes such the consumption of immense amounts of fossil fuels. as the shrinking of glaciers at Mount Kilimanjaro is As discussed in more detail in Chapter 2, science is a typical of the environmental problems we face today process, a “work in progress,” in which scientists publish and motivates our use of a holistic approach known as the results of their investigations based on the tools avail- Earth system science. In this approach, multiple fields are able and their best understanding of the data at the time. integrated in the study of Earth as a system. Within the As of 2012, for example, scientists working at Mount whole Earth system are many smaller, interacting com- Kilimanjaro were able to show, through a combination ponents that function as systems themselves but together of climate modeling and analysis of satellite data, that are dynamic parts that shape the greater entity. regional deforestation and drying had an insignificant Earth can be viewed as consisting of five major effect on the demise of the snows of Kilimanjaro in com- systems that continuously interact with one another parison to the effects of human-caused global warming.4 and with solar energy, gravitational energy, and the Why might melting glaciers matter? For Tanzania, internal heat energy of the planet to produce the cli- the loss of famed glaciers might not affect tourism, as mates and environments we experience at Earth’s surface hikers will still want to reach the highest peak in Africa.5 (Figure 1-2). From the bottom up, these are the geosphere Nevertheless, a cherished icon will be missed. For other (Earth’s rocky crust and mantle and its interior metallic locales where glaciers are shrinking, as in Pakistan core), the pedosphere (weathered, broken particles of and India, summer meltwater from glaciers is a critical rock capped with soil), the hydrosphere (water vapor, resource for farming and domestic use. The reservoirs streams, lakes, groundwater, and ice), the biosphere (liv- of water stored and released by glaciers serve much the ing organisms), and the atmosphere (air). These systems 6 PART I Introduction tend to permeate each other. Water, for example, resides not only in large pools such as oceans and rivers but also in the soil, in the air, and in living things. Similarly, living things reside on and in rocks, soils, bodies of water, and Reservoir Stock the atmosphere. Despite these overlaps, each “sphere” is an identifiable reservoir that can be viewed as an open, dynamic system into and out of which energy and mat- ter flow. What we mean by an open, dynamic system is discussed further below. An Earth system science approach provides not only a physical basis for understanding our world, but also the knowledge needed to attain sustainability, a long- term condition in which development meets the needs of the present without compromising the environment or the ability of future generations to meet their needs.6 Because environmental changes and geologic haz- ards result from the interactions of many Earth systems with one another, they need to be addressed by broadly trained Earth scientists with the ability to cross tra- ditional disciplinary boundaries. Relevant disciplines Flow rates (fluxes) include geology, ecology, chemistry, hydrology, soil sci- Figure 1-3 A system consists of reservoirs, which contain ence, atmospheric science, and climatology. As scientists a particular material, such as the water in this example of a learn more about the interaction of Earth systems, they sink. The content of the reservoir is the reservoir’s stock, and can make increasingly effective recommendations for the flow of that stock from one reservoir to another is called conserving resources, minimizing environmental deg- the flow rate, or flux. The stock is maintained in a steady state if the inflows and the outflows are in balance. radation, and reducing the potential for fatalities and destruction associated with geologic hazards. They also can make better predictions, as with the timing of the demise of glaciers atop mountains in response to climate time. The kitchen sink is a reservoir, the amount of water change driven by human action. that it holds at any given time is its stock, and the rates at  Glaciers on Mount Kilimanjaro, the tallest which water flows into and out of it are fluxes. mountain in Africa, have been shrinking rapidly since In nature, a lake operates in a manner similar to the early 20th century and are nearly gone; scientists the sink. The quantity, or stock, of water held by the estimate they will disappear within decades as a result lake depends on the flux of water into it from streams, of human-produced global warming. groundwater, and rainfall and the flux of water out  Earth system science integrates multiple fields in of it via streams, groundwater, and evaporation. One the study of Earth as a system within which there focus of the systems approach is to learn how systems are smaller, interacting components that function as stay in balance or change over time. We do this by mea- systems themselves. suring the fluxes of energy and matter into and out of a system and from one reservoir to another.  Primary Earth systems include the geosphere, pedosphere, biosphere, hydrosphere, and atmosphere. Types of Systems A system can be described as open, closed, or isolated, depending on how freely matter and energy can flow the concept of systems across its boundaries (Figure 1-4). An open system A system is a group of interrelated and interacting objects allows both matter and energy to flow in and out and is and phenomena consisting of reservoirs and fluxes. A the most common type in nature. A lake is an open sys- reservoir is a container that holds an amount of a par- tem because both matter (water molecules, sediment, ticular material. A kitchen sink, for example, holds a organic materials, gases) and radiant energy from the given volume of water that is maintained by pouring into Sun can enter and leave it. Closed systems are charac- the sink as much water as drains out (Figure 1-3). The terized by the ability to exchange energy but not mat- content of a reservoir, whether matter or energy, is ter across their boundaries, whereas isolated systems known as its stock. Fluxes are the movements of material have no interactions with their surroundings and allow and energy from one reservoir to another. Also called neither energy nor matter to cross. Although closed flow rates, fluxes are measured in amounts per unit of systems rarely occur naturally on Earth, for all practical CHAPTER 1 Dynamic Earth Systems 7

is the ability to do work, which is defined as a change Sun brought about when a force is applied. Energy is so Sun important to the operation of environmental systems Rain Solar Rain energySolar that it is sometimes treated as a system in itself. The energy manner in which changes to a system occur is known as a process; common processes that enact change in envi- Heat ronmental systems are the formation and subduction of energyHeat energy tectonic plates, volcanism, mountain building, erosion, and flooding. We will define and explore these processes in later chapters. Most Earth systems are dynamic. Life on Earth, for example, is a dynamic system in which processes Runoff such as growth and reproduction are powered primar- Runoff ily by energy from the Sun. The state of this system has changed with time in that the numbers and types of spe- cies on Earth have increased for several billion years and occasionally have plummeted during periods of major Lake water Lake water extinctions. In contrast, the Moon is a relatively static system in which little change occurs with time, and its (a) Open system appearance does not alter significantly with time. (a) Open system System Behavior In many dynamic systems, the flow of matter or energy or both into a reservoir is equal to the flow out. As a result, the stock of the material or energy remains con- stant with time even as the precise atoms making up that stock have changed, a condition known as steady state. The ocean, for example, is considered to be at approximate steady state because the total amount of water and dissolved substances it contains has changed relatively little over the past few thousand years. If we look further in the past, however, we find that the dynamic ocean system was not always at approxi- (b) Closed system (c) Isolated system mate steady state. About 20,000 years ago, sea level was (b) Closed system (c) Isolated system nearly 120 meters (m) lower than it is today because Figure 1-4 Systems are classified as open, closed, or much of the world’s water was frozen into extensive ice isolated depending on the nature of their boundaries and sheets during the full-glacial conditions prevalent at that interactions with their surroundings. (a) An open system allows matter and energy in and out. (b) A closed system, time. Between 16,000 and about 6000 years ago, Earth rare in nature, allows only energy in and out. (c) An isolated warmed, melting all but the Greenland and Antarctic ice system blocks matter and energy from entering or leaving. sheets and small remnants of once larger mountain gla- ciers. The ocean reservoir could not maintain its former steady state because its fluxes were no longer in balance; the inflow of water from melting ice was greater than the purposes we can consider the planet as a whole to be outflow of water through evaporation. As a result, the closed. Aside from the occasional meteorite and small stock of ocean water increased with time, resulting in a amount of cosmic dust that reach Earth’s surface and rise in sea level that slowed pace about 6000 years ago. the minor amounts of light elements that escape from It turns out that cyclical changes in Earth’s orbit around the outer atmosphere, Earth’s mass has been relatively the Sun have repeatedly led to oscillations of melting and constant for more than 4 billion years. While our planet growth of ice sheets, and as a consequence, oscillatory contains numerous examples of open and closed sys- rising and falling of sea level over tens to hundreds of tems, no system on Earth is truly isolated. thousands of years. Systems can be further described as dynamic Sometimes the flow of material into a reservoir or static. A dynamic system is one in which energy exceeds the flow out for an indefinite period of time. Such inputs cause the system to change over time, whereas a behavior can lead to different rates of growth, two com- static system is one in which no change occurs. Energy mon types of which are linear and exponential growth 8 PART I Introduction

9000 sphere, as shown in Figure 1-6a. A small cooling of the atmosphere can lead to the growth of ice. Unlike dark 8000 Linear growth surfaces, which absorb solar energy and convert it into Exponential growth 7000 thermal energy, the bright white surface of clean glacial Human population growth 6000 ice reflects 40 to 90 percent of the sunlight that strikes 5000 it. As a result, the atmosphere above the ice cools fur- ther, which promotes the formation of more ice, which 4000 reflects more light and leads to yet more cooling. The 3000 process of glacial ice formation thus promotes still more Population (millions) 2000 glacial ice formation. Reinforcing feedback processes are destabilizing in that they promote a cascade of events 1000 that propels the system toward accelerating change. 0 A balancing feedback is a process by which a 0 500 1000 1500 2000 change in one direction leads to events that reverse the Year direction of change. Balancing feedbacks are stabilizing Figure 1-5 Using population as an example, if growth were because they counteract the effect of the initial event linear, the number of people would increase steadily and in and help to regulate the system so that it maintains a proportion to the amount of time elapsed. In exponential steady state. For example, carbon dioxide is a green- growth, on the other hand, the increase with time depends on house gas, meaning that it absorbs energy given off by the size of the population, such that as the stock (i.e., number Earth’s surface and traps that energy in the atmosphere, of people) grows, the number of people that is added also grows over time. The actual rate of human population growth which warms our planet. Increased emissions of carbon over the past 2000 years, however, has been less gradual than dioxide from burning coal in power plants and gasoline the rate that would have resulted if human population growth in automobiles would logically lead to global warming. had been truly exponential since 1 ce. The picture is much more complex, however, because a warmer atmosphere holds more water vapor, which can result in more cloud cover. Clouds can reflect solar energy back into space before it reaches Earth’s surface (Figure 1-5). In linear growth, inflows and outflows have and is transformed into heat. The result is cooling, as fixed values and the entity growing increases in propor- shown in Figure 1-6b. Because an initial event that tion to time elapsed, resulting in a linear increase in the causes warming results in a process that leads to cool- stock of the reservoir over time. In exponential growth, ing, this chain of events is a balancing feedback process. on the other hand, the inflow depends on the amount Clouds also can act as reinforcing feedbacks because of material already in the reservoir, so that as the stock they can absorb the energy given off by Earth’s surface. grows, the inflow grows with it. Whether the feedback effect on climate is net reinforcing The world’s human population provides an example or balancing depends upon the clouds’ attributes (e.g., of nonlinear growth with time. The number of people droplet size and composition, whether ice or liquid) and alive could be viewed as a stock, with births and deaths their elevation above Earth’s surface. At present, global as the fluxes into and out of this reservoir of humankind. climate models do a relatively poor job of simulating Some aspects of human population growth are similar to cloud formation, so we cannot be certain that global exponential growth, in that the rate of growth depends warming will be offset by increased cloudiness. on the number of people alive to reproduce. As can be seen in Figure 1-5, however, the actual rate of increase  A system is a group of interconnected and in world population differs somewhat from that of an interacting objects and phenomena that can be exponential growth curve. Human population growth is separated into reservoirs of matter and energy, discussed in more detail in Chapter 7. connected by fluxes of matter and energy from one reservoir to another. Feedbacks in Earth Systems  In an open system, both matter and energy can As systems evolve over time, processes may arise that flow back and forth across the system’s boundaries. promote further change in the direction the system is In a closed system, only energy can flow across the moving, a phenomenon known as a reinforcing feed- boundaries. Isolated systems exchange neither matter back. One example of such a process is the ice-albedo nor energy with their surroundings. (albedo refers to how much incoming solar energy a  Dynamic systems are systems in motion. They surface reflects) feedback, which occurs as a result of display a variety of behaviors depending on the relative the interaction between Earth’s ice sheets and the atmo- sizes of their inflows and outflows, whether those flows CHAPTER 1 Dynamic Earth Systems 9

Ice reflects sunlight Expanded ice sheets Atmosphere back into space Atmosphere reflect more sunlight cools further cools into space

Ice sheet Expanding ice sheet

(a) REINFORCING FEEDBACK

CO2 from coal-burning Atmosphere Increased cloud cover power plant traps warms, holds reflects more sunlight Atmosphere reflected heat more water vapor into space cools

(b) BALANCING FEEDBACK Figure 1-6 (a) An expanding ice sheet shows how reinforcing feedback is a destabilizing process. (b) A simplified model of the greenhouse effect shows how balancing feedback is a self-regulating, stabilizing process.

are constant or changing, and whether they are fixed or rich organic matter and consists of solid, liquid, and dependent on the reservoirs to which they are attached. gaseous compounds. The hydrosphere is mostly hydro-  Most Earth systems are both open and dynamic. gen and oxygen combined to form water molecules in the gaseous, liquid, and solid states. The biosphere is  Reinforcing and balancing feedbacks either amplify composed mostly of the elements hydrogen, carbon, and or resist changes within and among Earth systems. oxygen in the solid, liquid, and gaseous states, and the atmosphere consists chiefly of free nitrogen and oxygen in their gaseous states. Earth’s Environmental Systems The Geosphere: Earth’s Metallic Earth’s five major systems will be treated in greater detail Interior and Rocky Outer Shell in later chapters of this book; here, let’s consider some Earth’s interior reflects the origins of the solar sys- basic information to begin exploring Earth’s processes tem 4.6 billion years ago. Based on observations of and the roles they play in environmental geology. Each numerous galaxies, astronomers hypothesize that our of the major systems differs from the others in composi- Sun and its orbiting planets formed from the collapse tion and physical properties (Figure 1-7). The outermost of a massive rotating cloud of gas and dust called a part of the geosphere, called the lithosphere (discussed nebula. Gravitational attraction drew the matter into below), is composed mostly of silicate minerals—solid a flattened whirling disk, with the bulk of the material compounds made primarily of oxygen and silicon with a at the center, where it developed into a star. Around few other elements including aluminum and sodium—as this central star, the remaining materials in the nebula well as iron and nickel. The pedosphere contains ele- gravitated into larger bits of rock, metal, liquid, and ice ments derived from the breakdown of rock and carbon- that coalesced into planets. 10 PART I Introduction

Lithosphere Pedosphere Hydrosphere Atmosphere Biosphere 80 70 60 50 40 30 Percentage 20 10 0

Iron Iron Silicon Silicon Oxygen Calcium Sodium Oxygen Carbon Calcium Sodium Oxygen Oxygen Oxygen Carbon Nitrogen Nitrogen Hydrogen Potassium Aluminum Hydrogen Aluminum Hydrogen Hydrogen Magnesium Magnesium Figure 1-7 Composition of each Earth sphere in numbers of contain substantial amounts of nitrogen—vital to plant atoms relative to a total of 100. (Because some atoms, such and animal life. In the hydrosphere, oxygen and hydrogen as hydrogen, are very small and light, a comparison based atoms combine to form water molecules, while the biosphere on weight of atoms rather than number of atoms would yield contains relatively large quantities of hydrogen, oxygen, and different results.) Both the atmosphere and the pedosphere carbon, the elements from which living organisms are formed.

During the first half-billion years of Earth history, the primordial Earth melted much, or perhaps all, of its so much debris was present in the solar system that mass, initiating a process that formed concentric lay- Earth was bombarded constantly by meteorites—bits ers with different chemical compositions and physical of rocky and metallic matter left over from the period characteristics. of planet formation. As the Sun became luminous, its radiation created solar winds, which forced much of Chemical Differentiation Iron and nickel, the two most the remaining debris out of the solar system. Since abundant heavy elements, sank toward Earth’s center then, Earth’s mass has remained essentially static, with to form the dense core of the hot, liquid Earth, located only minor additions from cosmic dust and occasional below about 2890 kilometers (km) (Figure 1-8). Lighter meteorite impacts. Earth’s chemical and physical char- elements, particularly oxygen, silicon, aluminum, magne- acteristics had just begun to take shape, however. sium, calcium, potassium, and sodium, floated upward. Soon after it formed from accreting debris, our When Earth’s surface began to cool and solidify, these planet began to heat up. Heating resulted from collision elements formed a rigid crust, which is Earth’s thinnest of debris, sinking of dense metallic elements into Earth’s and least dense rock layer, extending from the surface interior, and radioactive decay of chemical elements. to a maximum depth of 80 km. Between the core and The temperature of the interior of Earth rose to about the crust lies the mantle, a vast layer of rock rich in 2000°C, high enough to melt iron and nickel. Between oxygen, silicon, iron, and magnesium and therefore of a 4.5 billion and 4.4 billion years ago, the intense heat of composition and density intermediate between the crust

Chemically Distinct Layers

Crust (oxygen and silicon)

Core (iron and nickel)

Mantle (oxygen, silicon, iron, and magnesium)

(a) (b) (c) Figure 1-8 (a) Early Earth was a homogenous, rocky mixture center and light elements floated upward. (c) As a result, of mostly iron, oxygen, silicon, and magnesium; there were Earth is compositionally layered in concentric spheres from a no continents, oceans, or atmosphere. (b) During the process dense core rich in iron and nickel, to a thick mantle of mostly of differentiation that resulted from catastrophic melting oxygen, silicon, iron, and magnesium, and a light crust of of most or all of Earth’s mass, dense elements sank to the mostly oxygen and silicon. CHAPTER 1 Dynamic Earth Systems 11 and the core. Most of the rocky matter in Earth’s crust occurs varies around Earth and lies between 10 and and mantle consists of silicate minerals, which are solid 200 km below the surface. The overlying rigid layer of compounds formed largely from silicon and oxygen. crust and upper mantle is called the lithosphere, and the underlying ductile layer is known as the asthenosphere. Physical Differentiation In addition to the chemical dis- Since the lithosphere/asthenosphere transition occurs tinctions between Earth’s layers, there are also physical within the upper mantle, it is not associated with a com- distinctions that are the result of differences in the struc- positional change such as that which characterizes the ture of minerals and in the way they behave under dif- transition from the crust to the mantle. ferent conditions of temperature and pressure. Heating a The location of the base of the asthenosphere is solid makes it less dense and more malleable. Compress- not very well defined, but many geoscientists consider ing a solid makes it denser and more rigid. From Earth’s it to extend beneath the lithosphere to a depth of about surface to its inner core, both pressure and temperature 670 km. Below this level, increasing pressure results in increase (Figure 1-9). Within Earth’s crust, the geother- a layer of dense, more rigid rock known as the lower mal temperature gradient is about 25°C per km, although mantle or mantle mesosphere (from the Greek word the rate differs at various locations. In other words, if mesos for “in the middle”), which extends to about you descend vertically 1000 m into a mineshaft, you will 2890 km. Between 2890 and 5150 km, the residual tem- feel a temperature increase of about 25°C (77°F). perature from when Earth melted is so high that its outer Close to Earth’s surface, the rocks of the crust and core is still molten, despite the great pressure from the upper mantle are still fairly cool and behave in a brittle overlying rock. From a depth of 5150 km to Earth’s cen- fashion, meaning that they can break when subjected to ter at 6371 km, however, the pressure is so immense that stresses. Below some depth, however, the temperature is the inner core is solid, even though it is compositionally high enough relative to the pressure of the overlying rock similar to the outer core and at least as hot. to cause the solid mantle to become somewhat ductile, or plastic, meaning that the rocks are capable of slow Movements within the Geosphere The brittle lithosphere flowing movements. The depth at which this transition is broken into more than a dozen separate plates that

Physically Distinct Layers Atmosphere Lithosphere (cool, hard rock) 100 km Asthenosphere (weak, "soft" rock) Mesosphere (dense, rigid rock) Outer core (liquid nickel and iron) Increasing Inner core (solid nickel and iron) temperature and pressure Figure 1-9 Earth is zoned physically as well as chemically. The outermost 50 to 100 km, including the crust and top of the mantle, is very hard and rigid, so it can break; this is the 5150 km lithosphere. Below it lies the asthenosphere, a weak zone 2891 km Continental with the consistency of stiff crust Oceanic taffy. Under the asthenosphere crust lies the mesosphere, which is solid and hard, and below the mesosphere lies the liquid outer core. The inner core, although chemically similar Asthenosphere to the outer core, is solid and 200 km rigid because of the immense 50–100 km Lithosphere pressure in the center of Earth. 12 PART I Introduction

Eurasian Plate North Juan de Fuca American Philippine Plate Plate Anatolian Plate Plate Caribbean Arabian Plate Plate Cocos African Plate Plate Pacific Plate South American Indian-Australian Nazca Plate Plate Somalia Plate Sub-plate

Antarctic Plate

Figure 1-10 The theory of plate tectonics holds that Earth’s motion contributes to volcanism, earthquakes, and climate outermost layer is broken into large, moving plates. Their change, as well as many other Earth processes.

resemble a giant spherical jigsaw puzzle (Figure 1-10). gift shop directly above one of the largest caverns, now These plates slowly drift about on Earth’s surface because a popular tourist site in the Appalachian Mountains. currents of thermal energy rise from the hot interior of To the owner, the rock beneath his home seems perma- Earth, causing the weak, plastic asthenosphere underly- nent and unchanging, despite nearby sinkholes and the ing the plates to flow. The creation, jostling, and destruc- cave’s evidence that over millennia rock disintegrates tion of these lithospheric plates generate earthquakes and and changes form. Likewise, mountains seem fixed and volcanoes, as we will see in Chapters 2 through 5. eternal to many people, yet to the geologist they are Dynamic processes related to the motion, creation, evidence that young rocks have been pushed upward by and destruction of lithospheric plates are referred to as plate tectonic processes. tectonism, from the Greek word tekton, which means The continual creation, destruction, and recycling “builder.” Tectonic processes have been continuously of rock into different forms is known as the rock building, demolishing, and rebuilding Earth’s litho- cycle, and it is driven by plate tectonics, solar energy, spheric plates and surface features for at least the past and gravity (Figure 1-11). The rock cycle begins when billion years, and possibly ever since Earth’s crust tectonic forces drive molten rock from Earth’s mantle solidified from cooling magma nearly 4 billion years ago. toward the surface. The hot melt is less dense than the Since then, the plates have been moving across Earth’s surrounding, cooler mantle rock and rises buoyantly surface, breaking apart, colliding with one another, and through it, just as hot smoke rises through the cooler continuously changing the locations of continents, ocean air around it. As the hot melt rises, it cools and freezes basins, mountain belts, and island chains. The collision into igneous rock, so called from the Latin ignis, “fire.” of lithospheric plates typically causes one to dive beneath Water, wind, biological activity, and other environmen- another into the mantle, a process called subduction. tal stresses may weather the rock, dissolving and break- Catastrophic events such as earthquakes and volcanic ing it up into particles that eventually move downward eruptions occur in seconds or minutes, but the tectonic and accumulate in layers of sediment (from the Latin processes that give rise to these events in the lithosphere sedere, “to sink down”). Under sufficient pressure, operate on time scales of hundreds of thousands to mil- sediments harden into sedimentary rock. Environmental lions of years. forces may again disintegrate the rock. Alternatively, tectonic forces may either drive sedimentary and other The Rock Cycle In terms of a human life span, rock types of rock back into the mantle and remelt it into seems eternal. For example, the owner of land under- magma, or subject the rock to enough heat and pressure lain by many connected caves recently built a house and to transform it, without melting, into metamorphic rock CHAPTER 1 Dynamic Earth Systems 13

TRANSPORTATION DEPOSITION EROSION

Extrusive BURIAL AND UPLIFT AND EROSION igneous COMPACTION Sediments become of sedimentary, metamorphic, and rocks sedimentary rocks igneous rocks

REBURIAL

Intrusive igneous DEEP BURIAL AND rocks METAMORPHISM Subsurface MELTING produces igneous rocks Sedimentary rocks transformed into metamorphic rocks

Figure 1-11 Cyclical processes link different rock types raise them again during mountain building, as well as by at Earth’s surface. Minerals and rocks are altered by plate surface processes driven by solar energy, such as weathering tectonic processes that bury rocks deep in Earth and then and erosion.

(from a Greek word meaning “to change form”). All open system into and out of which move liquids, solid rocks on Earth can be classified as igneous, sedimentary, particles, and gases associated with the hydrosphere, or metamorphic. biosphere, and atmosphere. The oxygen, water, and acids abundant at Earth’s surface transform rock into pedospheric materials. Minerals weathered from rocks The Pedosphere: Where Soils Form and sediments are mixed with organic matter produced Atop the lithosphere lies the pedosphere, a layer of dis- by plants and animals to produce soil, the topmost part aggregated and decomposed (weathered) rock debris at of the pedosphere. However, with depth and distance the surface of exposed landmasses (Figure 1-12). The from the surface, the pedosphere gradually becomes prefix ped, derived from the Latin word for “foot,” is indistinguishable from the lithosphere. used to refer to this layer because it lies underfoot. The Some scientists have defined a part of Earth’s outer- composition of solid mineral matter throughout the most crust as the critical zone. This near surface zone is pedosphere is similar to that of Earth’s crust, consisting the locus of complex interactions among rocks, water, almost entirely of eight elements: oxygen, silicon, alumi- the atmosphere, and living matter. The name “critical num, iron, magnesium, calcium, sodium, and potassium. zone” derives from the fact that this zone regulates nat- Due to its unconsolidated nature, the pedosphere is an ural habitat and determines the availability of resources 14 PART I Introduction

Figure 1-12 Earth’s terrestrial surface is subject to weathering, a suite of disintegration and decomposition processes, because the geosphere is exposed to the atmosphere, hydrosphere, and biosphere. The result is the pedosphere, of which soil forms the topmost meter or so in the zone of most intense biologic activity. This view of a road cut in southern Washington shows many buried soils, or paleosols (light and dark subhorizontal bands), that record about a million years of pedosphere processes during changing climatic conditions. (Jim Richardson/Getty Images)

needed to sustain life.7 In a general sense, the terms combination of temperature and pressure at its surface critical zone and pedosphere refer to the same thing. that allows water to exist in all three states of mat- The total thickness of the pedosphere might be as ter: solid, liquid, and gas. Earth’s abundance of water much as 100 to 200 m in humid, tropical areas where gives the planet a shimmering, blue and white marbled rainfall and temperature are high and weathering pro- appearance when viewed from space (Figure 1-13). cesses extend deeply into the crust, or as little as 0 m Water permeates all of Earth’s surface systems, yet in a cold, dry desert. The soil part of the pedosphere its movement defines the hydrosphere, a zone about 10 typically measures only 1 to 2 m thick, yet all terres- to 20 km thick, extending from a depth of several kilo- trial agricultural activities and food production depend meters in Earth’s crust to an upper limit of about 12 km on this layer for nutrients and support. Like the geo- in the atmosphere. If all water on Earth (1.4 billion km3) sphere, soil is arranged in layers or “horizons,” with were evenly distributed, it would form a layer nearly the horizon poorest in organic matter at the bottom 3 km thick. Most of Earth’s water—97 percent—occurs and the horizon richest in organic matter at the top. The organic matter in soil contains essential elements that are vital to plant life, such as hydrogen, carbon, nitrogen, and phosphorus. Rates of soil-forming processes are affected by climate, rock type, organic matter, topography, and time. Most soil-forming processes operate on time scales of thousands to millions of years, and natural processes of soil erosion generally match rates of soil formation. Unfortunately, more rapid processes of soil destruction, such as erosion due to deforestation or poor farming practices, operate on much shorter time scales—years to hundreds of years. If the rate of destruction of soil exceeds that of formation, the soil stock, or reservoir, will become depleted with time, leading to lower soil fertility.

The Hydrosphere: Earth’s Distinguishing Characteristic Figure 1-13 Earth’s abundance of water, in all three states Rain, river flow, and waterfalls are phenomena that we of matter, gives the planet’s surface a marbled blue and white largely take for granted, but they do not occur on any appearance when viewed from space. This view, a photo other planet in our solar system. Earth is the only planet taken from the Moon by astronaut William Anders in 1968, orbiting the Sun with abundant water and the unique gave rise to the phrase “blue marble.” (NASA) CHAPTER 1 Dynamic Earth Systems 15 in the oceans and is salty because runoff from the con- eruptions still contribute some water vapor and other tinents brings with it dissolved rock matter in the form substances to the atmosphere and hydrosphere today, of ions (atoms or molecules with net electric charge). Of outgassing is much less pronounced than before much the 3 percent of water not in oceans, 2.7 percent occurs of Earth cooled to a solid state. in solid form as ice. The remaining 0.3 percent occurs as Water in its three states migrates through a vast freshwater on continents (rivers, lakes, and groundwa- global hydrologic cycle at different rates and over a ter) and as vapor in the atmosphere. range of time scales in Earth’s near surface environ- The hydrosphere started to form about 4.4 billion ments (Figure 1-14). As vapor in the atmosphere, water years ago during the period of global melting, when moves rapidly about the globe, on time scales of hours some oxygen and other light elements, such as hydro- to days and months. The movement of water in its gen and nitrogen, floated up from the molten Earth as liquid state in streams and oceans occurs more slowly, gases. Because of the gravitational attraction of Earth’s over periods of days to thousands of years. In its frozen mass, some of these gaseous elements—particularly the form, water in ice caps, ice sheets, and glaciers migrates heavier ones—were unable to escape to space. Rather, over periods of thousands to tens of thousands of years. they remained, some of them combining with one Relative to other liquids, water can store vast another, to form an envelope of gaseous matter around amounts of heat energy. In areas that receive large Earth. Although hydrogen is the lightest element that amounts of solar radiation, water stores solar energy as exists, it so readily combines with oxygen to form water heat and becomes warmer, whereas in areas that receive

(H2O) that much of it remained in this early atmo- less solar radiation, water releases its stored heat energy sphere. As Earth continued to cool, the water vapor and becomes cooler. As a result, the movement of water in the atmosphere condensed and rained down, filling from one reservoir to another and the circulation of the ocean basins. Little water has been added to or lost water within individual reservoirs such as the ocean from the surface of Earth since the first rains that fell affects local to global climatic conditions and helps to from the newly formed atmosphere. Although volcanic regulate Earth’s surface temperature.

Precipitation onto land Precipitation into oceans (119,000 km3 Atmospheric 3 /yr) water: (458,000 km /yr) Evaporation and 12,900 km3 transpiration from land EVAPORATION (72,000 km3/yr)

EVAPORATION TRANSPIR PRECIPITATION ATION

RUNOFF PRECIPITATION INFILT RATION SurfaceSurfS ac runoff and Evaporation from oceans groundwatergroundoou discha dwaw ter discha (505,000 km3/yr) (47,000(4 km rge 3/yr)

Continental water: 47,971,710 km3

Figure 1-14 Earth’s hydrologic cycle consists of the processes (and Oceanic water: fluxes) of precipitation, evaporation, 1,338,000,000 km3 transpiration, infiltration, and runoff from one reservoir to another. Primary reservoirs are the oceans, continental water (lakes, streams, wetlands, and groundwater), and the atmosphere. 16 PART I Introduction

1000 Figure 1-15 Temperature in the troposphere declines with altitude (which here is measured logarithmically) because the troposphere is heated from below by Earth’s surface. 500 Concentration of ozone (O3) in the stratosphere allows temperatures to rise, but in the mesosphere they fall again. THERMOSPHERE 200 Molecular oxygen (O2) in the thermosphere absorbs heat energy, so temperature rises with altitude. Clouds depict the Mesopause 100 location of water in the atmosphere. MESOSPHERE

Stratopause 50 Altitude (km) Ozone region The Atmosphere: Earth’s Envelope STRATOSPHERE 20 Tropopause of Gases 10 The Greek word atmos means “vapor,” and provides the name for the atmosphere, an envelope of gases that sur- TROPOSPHERE 5 rounds Earth’s surface. Part of this envelope can be seen from space as swirling clouds, large masses of air that 2 contain tiny droplets of water and ice crystals (see Figure 1-13). The atmosphere actually extends from just below Weather zone 1 the planet’s surface, where gases penetrate openings such as caves in the lithosphere and animal burrows in the Mount Everest 0 pedosphere, to more than 10,000 km beyond Earth’s -100 -50 050 100 surface, where gases gradually thin and become indistin- Temperature (°C) guishable from the solar atmosphere (Figure 1-15).

Figure 1-16 (a) Data from NASA’s Terra satellite can be yellows to oranges and browns are cropland, grasslands, used(a) to map land cover on Earth’s surface, showing the and shrublands, white is snow and ice, and gray is barren or distribution of ecosystems and land use patterns. These sparsely vegetated land. (b) The vertical extent of life on Earth maps aid scientists and policy makers involved in natural is referred to as the biosphere. The area below sea level is not resource management. In general, green colors are forests, drawn to scale. (a: Courtesy of Boston University and NASA GSFC) CHAPTER 1 Dynamic Earth Systems 17

The clouds of moisture so obvious in Figure 1-13 decreases rapidly to nearly zero just a few kilometers actually represent only a small portion of the gases in above Earth’s surface. the atmosphere as a whole (and are an example of the Venus and Mars are the most Earthlike planets in overlap between the hydrosphere and atmosphere). the solar system, yet they have atmospheres that consist Almost all the volume of gas in the atmosphere consists almost entirely of carbon dioxide. The difference between of nitrogen (78 percent), oxygen (21 percent), argon their atmospheres and Earth’s seems to be related to the (0.9 percent), and carbon dioxide (0.03 percent). Other existence of life, because the biological activity of plants gases, including neon, helium, nitrous oxide, methane, produces oxygen. Some of Earth’s oxygen combines and ozone, occur in trace amounts. The percentage of high up in the atmosphere to form a protective layer of water vapor at the base of the atmosphere varies consid- ozone. Other gases in the atmosphere store and transfer erably, from 0.3 percent on a cold, dry day to as much thermal (heat) energy at Earth’s surface, modulating the as 4 percent on a hot, wet day, but the water content planet’s temperature and producing the weather systems that move water evaporated from the ocean basins over land where it can fall as rain or snow. If Earth had no atmosphere, its surface temperature would be at least 9000 Occasional insects and 33°C cooler on average, or a chilly –18°C! a few other organisms

8000 The Biosphere: Where Life Exists The biosphere is a thin layer of life-forms that live in, 7000 on, and above Earth’s other environmental systems (Figure 1-16). An astonishing number of different spe- 6200 Limit of terrestrial animals cies of organisms are thought to exist on Earth—some 6000 Limit of higher plants 8 to 9 million.8 The majority depend on photosynthesis, 5500 Limit of human habitation the process by which green plants convert solar energy, carbon dioxide, and water into food energy. These 5000 organisms thus are confined to a zone in which they can 4500 Limit of cultivation receive solar radiation. This zone overlaps part of the 4000 atmosphere, the land surface (the top of the lithosphere and uppermost part of the pedosphere), and the illumi- Meters above sea level nated (euphotic) zone of water bodies (see Figure 1-16). 3000 The composition of the biosphere is similar to that of Earth’s other systems, but much closer to that of the hydrosphere than the lithosphere, as living cells 2000 are generally 60 to 90 percent water (see Figure 1-7). Like the atmosphere and hydrosphere, the biosphere is 1000 dominated by lighter elements. In fact, no elements with atomic numbers higher than 53 (iodine) are found in liv- ing cells except in trace amounts. By number of atoms, 0 the biosphere consists of 99 percent hydrogen, oxygen, Euphotic carbon, and nitrogen, and its high percentage of carbon zone distinguishes it from all the other major Earth systems. 200 Carbon atoms form strong bonds with one another, enabling them to develop elaborate chains of unlimited 1000 length, as well as rings and branching structures that can attach to other atoms and molecules. These com- plex organic (i.e., carbon-containing) macromolecules 1500 sometimes consist of thousands of atoms and form more compounds than any of the other 102 elements on Earth, Meters below sea level which is one reason most chemistry students consider 10,000 organic chemistry to be among their hardest courses! The fundamental organic molecules are carbohydrates, Deepest parts of the ocean floor fats, proteins, and nucleic acids. All fossil fuels (oil, coal, 10,850 and natural gas) were formed from organic molecules that originated in the biosphere but were buried by

(b) 18 PART I Introduction sediments to become part of the lithosphere after the  The pedosphere, generally less than 100 to 200 m death of the organisms that produced them. thick, is the entire layer of weathered rock debris and In the early history of the planet, oxygen in the atmo- organic matter at the surface of exposed landmasses. sphere was insufficient for animal life to exist. Instead, The relatively organic-rich upper meter or so of the Earth’s primitive atmosphere contained large amounts pedosphere, called soil, provides the nutrients essential of methane (CH4), but these molecules were split apart to agricultural production on land. by solar radiation, and the carbon combined with the  Earth’s gaseous atmosphere and largely liquid little oxygen that existed to form carbon dioxide (CO2). hydrosphere formed by outgassing of volatile elements Early life-forms, including bacteria, blue-green algae, and from the early molten Earth as it differentiated phytoplankton, used the CO2 for metabolism, producing into physically and chemically distinct layers. Both oxygen as a by-product. Fossil evidence indicates that atmosphere and hydrosphere serve to modulate these early photosynthesizers existed by at least 3.5 bil- Earth’s surface temperatures, keeping them within a lion years ago and perhaps earlier. With time, the amount fairly limited range, by transporting heat absorbed of CH4 in the atmosphere decreased, while the amounts from the Sun from one location to another. of CO and free oxygen increased. The continued evolu- 2  Because of biological activity, Earth’s atmosphere tion and spread of plants—which increased amounts of is rich in oxygen compared with the atmospheres of oxygen in the atmosphere—enabled oxygen-breathing nearby planets. In turn, Earth’s atmosphere shields animals to evolve. Some of these animals (herbivores) the biosphere from harmful UV rays from the Sun by relied on consuming plant matter for energy, while oth- efficiently absorbing them. ers (carnivores) preyed on other animals to fuel their own metabolism. Increased levels of oxygen in Earth’s  The biosphere of Earth is unique in our solar atmosphere also allowed for the formation of ozone in system and formed soon after the early atmosphere the upper atmosphere, which shields organisms from and hydrosphere developed. It is an exceptionally thin ultraviolet (UV) radiation given off by the sun. UV radia- layer, but it contains millions of different species that tion can lead to sunburn, genetic defects, and cancer, so have evolved since the earliest, simple forms of life. only when the atmosphere contained enough ozone could animals evolve on land. Earth is the only planet in the solar system known Earth’s energy System to have life. The role of water in the evolution of life is Energy can be considered a sixth environmental system. essential, in part because of the many special properties Earth generates energy in its own interior and receives of the water molecule, such as its high heat capacity. Life energy from the Sun. Energy from these two sources, began in the oceans, and those life-forms that moved plus small amounts generated from the gravitational onto the continents carried their ocean environments attraction between the Moon, the Sun, and Earth, with them, in their cells. It should come as no surprise, flows through all the other Earth systems (Figure 1-17). then, that the human body is nearly two-thirds water, Since energy is the ability to do work, all processes and the water flowing through the bloodstream and and changes would cease if Earth received no energy. bathing the cells carries on the task of the ocean by Earth’s access to energy and ability to use it make the supplying the body with nutrients and removing waste planet a dynamic system. products. To stay alive, terrestrial creatures require a supply of water and, consequently, are greatly dependent on the processes that involve water on land. In this sense, States of Energy the biosphere and hydrosphere are intimately connected. Energy exists in several states, including kinetic, poten- tial, and thermal. Kinetic energy (KE) is the energy of a  The geosphere formed by accretion of rock and body in motion and is defined as KE = 0.5 mv 2, where metal debris associated with the nebula that became m is the mass of the body and v is its velocity. Potential our solar system. Subsequent melting of this material energy (PE), in contrast, is the energy of a body that led to its chemical differentiation into the layers that results from its position within a system. Potential energy became the inner and outer core, mantle, and crust. is related to the gravitational attraction between bodies  The lithosphere is a rigid, rocky layer of crust and is expressed as PE = mgh, where m is the mass of and upper mantle 10 to 200 km thick that overlies the body, g is acceleration of the body due to the gravi- a more ductile layer, the asthenosphere, within the tational force, and h is the relative height of the object. mantle. The lithosphere is broken into plates that float Boulders perched on a hillside contain potential above the asthenosphere and move when heat from energy as long as they are at rest. If a process—per- Earth’s interior induces flow in the asthenosphere. haps an earthquake—causes the ground to give way, The outermost part of the lithosphere is a chemically the boulders will roll downhill under the influence of distinct layer called the crust. gravity. Before the rockfall begins, the rocks contain CHAPTER 1 Dynamic Earth Systems 19 potential energy proportional to their distance from Energy can be transferred between bodies in the Earth’s center. As they roll and bounce down the slope, universe, as in the transfer of thermal energy. All bodies their potential energy is converted into kinetic energy contain internal thermal energy, which arises from the and thermal (heat-related) energy (due to the motion random motions of their atoms. The term heat describes of atoms). By the time the boulders settle at the bottom the transfer of thermal energy from one body to another. of the hill, all their potential energy has been converted In a body receiving thermal energy, the added energy into thermal energy as a result of frictional resistance. causes the atoms in the body to speed up. The tempera- This example demonstrates a fundamental principle: ture of a body is a measure of the average speed at which Energy is completely conserved when work is done. its atoms move. When heat is transferred to a body, its While the energy has undergone a change in state (from temperature rises because its atoms move faster. potential to kinetic to thermal), the amount of potential Energy is measured in many different units, includ- energy before the work was done equals the amount of ing foot-pounds, British thermal units (Btu), and electron kinetic and thermal energy expended to do the work. volts, but the most common convention among Earth

Solar energy

Atmosphere

Hydrologic Cycle Biochemical Cycle Precipitation Gravitational pull from Sun Biosphere and Moon Rock Evaporation Cycle Volcano Photosynthesis

Tides Pedosphere

Lithosphere Rising magma

Ocean circulation

Heat flux Geothermal energy

Figure 1-17 Energy is the driving force for all processes on Earth. Matter and energy flow cyclically through the hydrosphere (hydrologic cycle), biosphere (biochemical cycle), atmosphere, pedosphere, and lithosphere (rock cycle) at Earth’s surface. 20 PART I Introduction scientists is to measure energy in either calories or joules. Earth’s surface is called insolation, and the amount One calorie (cal) is defined as the amount of energy received varies around the world as well as throughout needed to raise the temperature of 1 gram of liquid each day and season (Figure 1-19). It is large during water by 1°C and is equivalent to 4.184 joules (J). When midsummer at midlatitudes, for example, and relatively contemplating how many calories our food contains, we small at the same locales in winter. During photosynthe- refer to Calories (upper case C), each of which is equiva- sis, plants combine solar energy with water and carbon lent to 1000 cal, or 4184 J. A candy bar, for example, dioxide to produce the carbohydrates on which the contains about 400 Calories, or 400,000 calories. The whole biosphere depends for food and fuel. When we human body (typically 50 to 80 kg) needs to maintain a burn wood in a campfire, we release this stored chemical temperature of 37°C (98.6°F). Because about 70 percent energy, which we then can use to warm ourselves or to of the human body is water, much of its caloric energy is cook food. used to maintain the temperature of water at 37°C. The Under the proper geologic conditions, some biomass lower the temperature of ambient air, the greater the may be converted to oil, coal, or natural gas. These sub- amount of calories needed. stances are called fossil fuels because they come from plants and animals that lived many tens to hundreds of millions of years ago. When we drive cars, we are using Sources of Energy solar energy that is many tens to hundreds of millions Three sources of energy, two internal and one external, of years old! The amount of energy stored within fossil power all the processes on Earth. Our planet receives fuels is extremely small compared with the amount of 1.73 × 1017 watts (W; 1 W = 1 J/sec) of power (energy solar energy received by Earth, because less than 1 per- per unit of time) from the Sun, which amounts to cent of incoming solar energy is converted to chemical 99.98 percent of its total energy budget. Smaller amounts potential energy by plants. In less than a single month, of energy are created within Earth through spontaneous Earth receives as much energy from the Sun as is stored in radioactive decay of some elements in rocks, and some known fossil fuel reserves. Because fossil fuels that were residual heat from Earth’s formation still exists. Together easiest to reach and extract have diminished, solar energy these amount to 32 × 1012 W of power. An even smaller is an increasingly important energy source. amount of energy (3 × 1012 W) is supplied by the gravita- tional attraction within the Earth–Sun–Moon system and Earth’s Internal Energy Earth makes its own supply drives the movement of ocean water as tides. of internal energy through the spontaneous decay of certain elements in minerals. In this process, known Solar Energy In the Sun, hydrogen atoms at extremely as radioactive decay, parent atoms convert to other high temperatures and pressures combine to form helium atoms by losing or gaining subatomic particles in their atoms in a process called fusion. This reaction releases nuclei (discussed more fully in Chapter 6). This process radiant energy that is transmitted through space in all releases relatively small amounts of thermal energy that directions, and Earth intercepts some of this energy, are transferred through the surrounding rock. warming the atmosphere, oceans, and land surface Although much smaller than the influx of solar (Figure 1-18). The amount of sunlight that reaches energy, it is Earth’s internal heat energy that drives

The atmosphere reflects some RERADIATED energy back out to space SOLAR ENERGY

INCOMING SOLAR ENERGY

Figure 1-18 Some solar energy is reflected, and some is Clouds and greenhouse gases absorbed and reradiated to outer in the atmosphere space. Inflow of energy is equal to trap some of the heat outflow, keeping Earth’s energy budget balanced. CHAPTER 1 Dynamic Earth Systems 21

2 W/m W/m2

0 275 550 Figure 1-19 Insolation is the amount0 of sunlight that 275(W/m2). Note how little550 sunlight is received by the south reaches Earth’s surface during a given time period, as shown polar region during July, which is winter for the southern here for July 1, 2012 through July 31, 2012. It commonly is hemisphere. (NASA) measured in units of watts per square meter of land surface

lithospheric plate motions, which in turn result in earth- is approximately equal to the amount of energy flow- quakes and volcanism. As plates move, energy can be ing out of the system. Because the Sun supplies all but stored in rocks, much as energy is stored in an elastic 0.02 percent of Earth’s energy, the inflow and outflow of band by stretching it. When such rocks finally snap, solar energy is a close approximation of Earth’s energy as would a band that had been stretched too far, the budget. An energy budget, like a monetary budget, is amount of energy released can be quite large. The 2004 an accounting of inputs and outputs, and a steady-state Indonesian earthquake that led to the Indian Ocean condition is like a balanced financial budget. tsunami, in which more than 275,000 people perished, As shown in Figure 1-18, more than half of all incom- released about 1.1 × 1017 J of energy, equivalent to more ing solar radiation is returned to space before doing any than 1500 nuclear bombs the size of those dropped on work at Earth’s surface. About 25 percent is reflected Hiroshima, Japan, during World War II. to space by particles or clouds in the atmosphere and another 5 percent is reflected by Earth’s surface, while Gravitational Attraction Earth derives energy from grav- 25 percent is absorbed by clouds and the atmosphere. itational attractions between itself and the Moon, the This leaves about 45 percent of incoming solar radiation Sun, and the other bodies in the solar system. Although to be absorbed by Earth’s surface. About two-thirds of minor compared even with Earth’s internal energy, the this absorbed energy (29 percent of the original incom- energy derived from gravitation causes both Earth’s crust ing radiation) is cycled through Earth’s environmental and ocean water to change shape over time in a regular systems and reradiated, while one-third (16 percent of fashion, resulting in rock tides as well as ocean tides. the original incoming radiation) is converted to heat (thermal energy). The wavelengths of reradiated energy are longer than those of the original (or reflected) solar Energy Budget of Earth radiation. Because clouds and greenhouse gases such as The energy system of Earth is in a steady-state condition; carbon dioxide trap energy at longer wavelengths rela- the amount of energy received by the whole Earth system tively easily, reradiated energy has a strong influence on 22 PART I Introduction

Earth’s climate. In Figure 1-18, you can see that inflow of solar energy is equal to outflow, with 30 percent of Global Energy Consumption the inflow leaving Earth as reflected energy and 70 per- 150 cent leaving as reradiated energy. Earth’s energy budget Crude oil is balanced. Coal Natural gas Biofuels, wood, charcoal Human Consumption of Energy 100 Nuclear Hydro Humans have devised many methods to harness energy. For millennia, people have used wood for heat and wind and moving water to grind grains and process other commodities in mills; more recently, a variety of 50 fossil fuels and other energy sources generate electricity (Figure 1-20). In Iceland, a country located on an active plate boundary with volcanism and high heat flow at the surface, the dominant energy source is geothermal. Global energy consumption (exajoules) Worldwide, however, fossil fuels (largely coal, crude 0 oil, and natural gas) have become the most important 1800 1850 1900 1950 2000 sources of energy during the past century. Year The total amount of energy used has increased with time even though the dominant energy sources have var- Figure 1-20 Global energy consumption has increased ied. In 2011, humans used a staggering 487 exajoules rapidly during the past two centuries, fueling the Industrial Revolution that continues to spread to countries worldwide. (an exajoule equals 1018 joules) of energy worldwide, First coal and then two other fossil fuels, crude oil and natural much of it to generate electricity. As a consequence, our gas, rose in prominence since the late 19th century. planet—particularly due to its more populated areas— resembles a well-lit black marble at night (Figure 1-21). The amount of incoming solar energy is roughly  Solar energy contributes 99.98 percent of the 12,000 times that of the world’s current energy con- energy that drives processes on Earth, but Earth’s sumption, but relative to the amount available, little internal thermal energy drives plate tectonics, while solar energy has been collected and used thus far. Most the gravitational attraction among Earth, the Moon, industrialized nations rely heavily on fossil fuels—oil, the Sun, and other bodies in the solar system drives coal, and natural gas—to power cars, heat homes and the tides. water, and generate the electrical energy needed to run various appliances and utilities. However, Germany,  Earth’s energy system is in a steady state; the Spain, and the United States currently are making sig- amount of energy received by the whole Earth system nificant advances in technologies in order to use more is approximately equal to the amount of energy solar energy for heating and electricity. flowing out of the system. Of all the different sources of energy available to the  Since the early 20th century, fossil fuels, largely world, fossil fuels satisfy about 81 percent of total energy coal, crude oil, and natural gas, have been the primary demand. However, the type of fuel used and the per cap- sources of energy consumed by humans. ita use of energy vary markedly from country to country,  as well as from region to region within countries. As World energy use is increasing over time, and was of 2010, the United States used more total energy than 487 exajoules in 2011. any other country in the world except China. With only about 5 percent of the world’s population, the United States consumes about 19 percent of the world’s annual Human population and primary energy demand (98 quadrillion Btu in 2010). In 2010, China surpassed the United States to become the earth system boundaries world’s leading consumer of energy, using 20.3 percent Several related issues have contributed to a grow- of the total. Still, its far greater population means that ing environmental awareness since about the mid- China’s per capita energy consumption remains lower 20th century, foremost among them population growth than that of the United States. and spreading industrialization. With population and  Three sources of energy are responsible for all industrial growth come ever-increasing demands for processes on Earth: solar energy, internal thermal such resources as fuel, land, and clean freshwater. energy, and gravitational energy. Environmental degradation is the nearly inevitable result CHAPTER 1 Dynamic Earth Systems 23

Figure 1-21 A growing population worldwide with access (left: IM_Photo/Shutterstock; right: NASA Earth Observatory Image to electricity and high population densities in cities gives by Robert Simmon, using Suomi NPP VIIRS data provided courtesy of Earth a nightly glow that resembles a well-lit black marble. Chris Elvidge [NOAA National Geophysical Data Center])

of these pressures and demands. In addition, although 2.2 percent to about 1.1 percent in 2012. As fertility and people always have lived in areas prone to earthquakes, mortality rates change, population growth rates vary. floods, and other geologic hazards, the increasing num- The current slowdown in population growth rate is a bers of people living in these areas heightens the poten- result of falling levels of fertility. tial for human disaster. Despite a decreasing rate since the 1960s, popula- tion growth still has considerable momentum because of the large number of people on Earth. Even at the rela- Human Population Growth tively low growth rate of 1.1 percent, another 75 million As the world’s population passed 7 billion sometime in people are added to Earth’s population each year. Most late 2011 to early 2012, concern continued to mount population growth is occurring in developing rather than over how many more people would be added in the already industrialized countries. Various predictions of future. Human population growth has been difficult to the world’s future population range from 6 to 16 billion predict accurately because it depends on a large number people during the next century. of variables—from advances in agriculture, sanitation, What are the consequences of more than 7 billion and medicine to the influences of culture, religion, people on Earth and a growing population with a desire and medical practices. The rate of human population for higher standards of living? To date, consequences growth has varied throughout human history, but its have included increasing use of resources (e.g., miner- most striking feature is marked growth into the billions als, fertilizers, and water), consumption of energy, and in the last few hundred years (see Figure 1-5). production of wastes. Unfortunately, they also have The human population has grown in a manner included a growing number of extinctions of plant and somewhat similar to exponential growth. It took 2 mil- animal species. lion years of human history to add the first billion peo- More people are living in cities; in fact, more than ple, 130 years to add the second billion (around 1927), 50 percent of people live in urban areas, and cities 30 years to add the third (1960), 15 years to add the are increasingly larger. In the early 20th century, only fourth (1975), and 12 years to add the fifth (1987). 20 percent of the population lived in urban areas. As a World population reached 6 billion in 1999 and 7 bil- result, population density, not just population, is increas- lion in 2011–2012, with each extra billion added after ing (Figure 1-22). 12 years. Since the 1960s, annual growth rates for The world’s first megacities, those with more than world population have declined from a high of roughly 10 million people, developed in the mid-20th century 24 PART I Introduction

persons/km2

110 100 1000 10,000

persons/km2

110 100 1000 10,000 Figure 1-22 World population density on average is about are all located in Asia: Tokyo, Japan; Jakarta, Indonesia; 50 people per km2 of land area, excluding Antarctica, but can Seoul, South Korea; and Shanghai, China. (Image by Robert be as high as 40,000 people per km2 in megacities. Note that Simmon, NASA’s Earth Observatory, based on data provided by the the four most densely populated places are Singapore, Hong Socioeconomic Data and Applications Center [SEDAC], Columbia Kong, Bahrain, and Bangladesh. The four largest megacities University)

(New York City was the first). Today there are 28 mega- A resource is anything we get from our environ- cities. Three are located in North America: Mexico City, ment that meets our needs and wants. Some essential New York City, and Los Angeles. In some of the world’s resources, including air, water, and edible biomass (plant most crowded cities, population densities are as high and animal matter), are available directly from the envi- as 40,000 people per km2 (about 100,000 per mi2). In ronment. Other resources are available largely because contrast, for Earth’s land area excluding Antarctica, the we have developed technologies for exploiting them. average population density is about 50 people per km2 These include oil, iron, and groundwater. In general, (130 per mi2). people in affluent and highly industrialized countries use far more resources than are required for basic survival. Resources and Sustainable The United States, for example, has only 4.8 percent of the world’s population, yet it consumes about 33 percent Development of the world’s processed nonrenewable energy and min- Does Earth have some finite “carrying capacity,” some eral resources. threshold number of people beyond which it cannot Resources are classified into three major types sustain the human population with clean air, clean according to their degree of renewability: potentially water, and adequate nourishment? If so, then at or renewable, nonrenewable, and perpetual (Figure 1-23). below this population, humans could lead a sustainable Potentially renewable resources can be depleted in the existence, one in which development meets the needs short term by rapid consumption and pollution, but of the present without compromising the environment in the long term they usually can be replaced by natu- or the ability of future generations to meet their needs ral processes. The highest rate at which a potentially as well.6 Although perhaps impossible to determine renewable resource can be used, without decreasing a threshold population size with certainty, we realize its potential for renewal, is its sustainable yield. If that both the number of people and the resources they the sustainable yield is exceeded, the base supply of consume are central to evaluating the possibility of the resource can shrink so much that the resource long-term sustainability. can become exhausted—used up. Soil formation, for CHAPTER 1 Dynamic Earth Systems 25 example, occurs at rates of about 2 to 3 centimeters (cm) per thousand years, making it a potentially renew- able resource. Unwise farming practices, however, can cause soil loss of 6 to 8 cm per decade. Soil cannot be renewed at this rate because its formation is depen- dent on plants and soil moisture, both of which are gone once the soil itself is lost. In some parts of the world, all soil has been removed and so is essentially nonrenewable. Nonrenewable resources, such as fossil fuels and metals, are finite and exhaustible. Because they are pro- duced only after millions of years under specific geologic conditions, they cannot be replenished on the scale of a human lifetime. The formation of oil, for example, requires that buried plant and animal matter be sub- jected to tens of millions of years of squeezing and heat- ing by geologic processes, a rate of formation so slow that the resource cannot be considered renewable. (a) Some nonrenewable resources, such as copper and iron, can be recycled or reused to conserve the supplies. Recycling involves collecting, melting, and otherwise reprocessing manufactured goods. Resources com- monly recycled include aluminum, paper, and iron. Reuse involves repeated use of manufactured goods in the same form. Glass bottles commonly are reused. Other resources, such as fossil fuels, cannot be recycled or reused because their combustion during energy production converts them to ash that falls to Earth, exhaust fumes that go into the atmosphere, and heat that ultimately leaves Earth as low-temperature radia- tion. Nonrenewable resources become economically depleted when so much—typically about 80 percent of the resource—is exploited that the remainder is too expensive to find, extract, and process. Oil buried in (b) rocks deep beneath the Antarctic ice sheet, for example, would be far more expensive to find, extract, and refine than oil trapped beneath the shallow salt beds of Texas. Perpetual resources are those that are inexhaustible on a human time scale of decades to centuries. Examples include solar energy—which has fueled numerous reac- tions and processes on Earth for its entire 4.6-billion-year history, heat energy from the interior of the Earth, and energy generated by Earth’s surface phenomena such as wind and water flowing downhill. One of the most significant changes in human history is the increased demand for, and use of, energy. Before the Agricultural Revolution started about 10,000 years ago, people’s energy demands were primarily for food. Figure 1-23 (a) Soil, a potentially renewable resource, On average, a person uses about 10 megajoules (MJ) (c)is put at risk by the demands for food associated with of energy (2000 to 3000 food calories) per day, and for worldwide population growth. (b) New technologies and early humans this was the bulk of their energy needs. By ever-larger machines are used to mine coal, a nonrenewable 2011, however, daily per capita energy consumption in resource, at increasing rates. (c) Solar energy is a perpetually the United States was more than 900 MJ, spent mostly in available resource. Reflective panels can focus this energy industry, transportation, and temperature control. Much on a “power tower” where it is converted into electricity. of this energy consumption goes toward the production (a: Hopsalka/Shutterstock; b: J. van der Wolf/Alamy; c: Felipe and transport of food. Rodriguez/Getty Images) 26 PART I Introduction

4 in automobile engines and industrial power plants and released into the atmosphere. These molecules react with Perpetual gases and water vapor in the atmosphere to produce resources 3 smog and acidic rainwater, both of which make the air Potentially renewable and water unhealthful, causing damage to plants, fish, resources land animals (including people), and even stone. 2 Wastes are unwanted by-products and residues left from the use or production of a resource. Early

Production rate human habitation sites are often identifiable by the 1 Nonrenewable piles of waste left behind, including shells, bones, char- resources coal from fires, and broken pottery. Sometimes, early humans reused wastes in innovative ways, building 0 huts from animal bones, for example. People tended 1850 1900 1950 2000 2050 2100 2150 to move on when their waste piles became substantial, Time (years) although substantial amounts of human detritus can Figure 1-24 As demand for a resource increases, more be found beneath cities that date to ancient times, such of it is produced. For nonrenewable resources, production as Rome and Paris. Most waste piles made by small cannot increase forever, because ultimately the resource bands of people were biodegradable, so they did little will be exhausted. Renewable resources can provide lasting long-term environmental damage. In contrast, modern supplies if the rate of production is similar to the rate of humans in industrialized societies have much higher renewal. Only for perpetual resources is unlimited increase in production possible. population densities and technologies that enable them to concentrate large amounts of waste, some of which is toxic and infectious, in small areas.

Production and consumption of renewable, non- A Model of Environmental Impact Scientists have devel- renewable, and perpetual resources follow different oped a simple model of environmental degradation and growth curves (Figure 1-24). For potentially renewable pollution to assess the environmental impact of human resources, such as wood, the curve is similar to that of populations. In this model, the extent of the environmen- the growth of a biological population: Production rises tal impact depends on three variables: (1) population, exponentially for some time, and then levels off at a (2) per capita consumption of resources, and (3) the state of no growth that is equal to the rate of replenish- amount of environmental degradation and pollution per ment. For nonrenewable resources, such as fossil fuels unit of resource used: and minerals, production rates rise exponentially, reach a maximum value, and then decline exponentially as Number × per capita × degradation supplies approach zero. For perpetual resources, such as of people consumption and pollution solar energy, production can rise exponentially for essen- of resources per unit of tially unlimited time periods. It is clear that humans can resource used rely on nonrenewable energy resources only for a limited = environmental impact time, before turning to a perpetual source of energy. Environmental degradation due to overpopulation Pollution, Wastes, and occurs when a large group of people has insufficient resources. Although the group’s per capita consump- Environmental Impact tion of resources may be low, the amount of environ- Many of the world’s environmental problems are mental degradation can be high because the group must related to the wastes produced by human activities. To scavenge all available resources in order to survive. a greater or lesser extent, all humans produce wastes Environmental protection means little in the face of that pollute and degrade air, water, and land. Pollution starvation. In parts of northern and western Africa, for is the contamination of a substance with another, unde- example, large areas of land have been denuded of most sirable material. Environmental pollution results when vegetation, and famine has occurred repeatedly in the a pollutant degrades the quality of an environment. 20th century. Common pollutants in urban areas are sulfur diox- Degradation due to overconsumption occurs when a ide (SO2) and oxides of nitrogen (NOx, molecules with small number of people use resources at such a high rate one or two atoms of oxygen and one atom of nitrogen), per person that they cause very high levels of resource all of which are created by combustion of fossil fuels depletion and pollution. Ranking first among modern CHAPTER 1 Dynamic Earth Systems 27 examples is the United States, which has less than 5 per- cent of the world’s population but consumes more than 33 percent of nonrenewable energy and mineral resources and produces more than a third of the world’s pollution. As a result, despite its relatively small population, it has a tremendous environmental impact. In contrast, China has about 19 percent of the world’s people, about four times as many as the United States, but causes less than one-tenth (or 10 percent) the environmental damage of the United States. For this reason, it sometimes is noted that the environmental impact of one person in the United States is equivalent to that of many more persons in developing nations, such as India, where resource use and pollution per person are much smaller. As nations develop, the extent of their environmen- tal impact due to consumption changes rapidly. In the 19th century, for example, during a rush to settle all Figure 1-25 In China, environmental pollution and parts of the United States, forests were destroyed to clear degradation are extreme as a result of a large population, fields for farming, and many other sites were severely rapid industrialization, and heavy reliance on burning coal for degraded by destructive mining practices. Since then, power. (Bernard Wis/Paris Match/Getty Images) increasing environmental awareness in the United States has resulted in significant efforts to prevent such degra- dation in future development. Likewise, as China enters the global market of free trade and as other nations 1960s to 1.73 percent in the late 1970s, to 1.6 percent become more developed, it is likely that the nature of in the 1990s, and to 1.1 percent as of 2012.9 This con- their environmental impacts will change. sistent drop in population growth rates is encouraging, The third component of environmental degrada- but rates must drop in many more nations in order to tion is pollution per unit of resource used. In some level off global population growth. cases, the amount of pollution produced per unit of Minimizing environmental degradation caused by resource used is so high that extreme environmental consumption can be achieved through reduced resource degradation occurs. For example, the increased use of consumption per person and coordinated efforts to coal for energy in China at a time of rapid industrial recycle or reuse products. Minimizing environmental growth is resulting in substantial air pollution prob- degradation caused by pollution can be achieved through lems (Figure 1-25). Industrialized countries in Eastern environmental regulation and the use of more efficient Europe and the former Soviet Union have amassed a technologies to minimize the amount of waste (much of legacy of environmental pollution that will affect their which produces pollution) generated during industrial citizens for decades to come. and resource extraction processes. Efforts to develop waste minimization procedures for different industries Minimizing Environmental Impacts Solving the problem are now the focus of research and testing at many univer- of environmental degradation caused by rapid popula- sities and industrial labs. Some new factories in Denmark, tion growth is essential, but the solution is not simple. for example, produce nearly zero waste, designing their Increasing access to medicine and sanitary supplies of activities so that waste products from one stage are used water worldwide has reduced death rates and thus con- as input to other stages of the manufacturing process. tributed to the rapid population growth of the past few As environmental regulations become more strin- centuries. As a result, reducing population will require gent, companies are developing new technologies to a change in the birth rate. Solutions for reducing birth extract and process resources that produce fewer wastes rates are related to education and standards of living, than ever before. New methods to mine copper that among other things. In most countries of the world, the require no excavation have been developed in Arizona higher the level of education and standard of living of and now are in use elsewhere (see Chapter 4). The cop- women, the more likely women are to use birth control per is removed from sediments and rocks by solution methods and have fewer children. techniques, so that heating and melting of rocks to sepa- Reduced birth rates in most developed nations, as rate precious metals are not required. The result is a sub- well as in China and India, caused worldwide popula- stantial (or nearly complete) reduction in the production tion growth rates to drop from 2.06 percent in the late of fumes, noxious gases, and ash that once was typical 28 PART I Introduction of the exhaust material belched from smokestacks at Risk refers to the magnitude of potential death, mine sites. injury, or loss of property due to a particular hazard. The risk of death in the United States each year by an earth- Natural Disasters, Hazards, quake or volcano (less than 0.1 death per million people) is far less than the risk of death by automobile accidents and Risks (120 deaths per million people) or fires (11 deaths per In contrast to environmental issues associated with million people). Different regions have different risks; population and pollution are geologic hazards that in California, the risk of death by earthquakes and vol- exist regardless of human activity but can put people in canoes is greater than it is in Ohio or Kansas. In fact, harm’s way as a result of human activity. For example, much of the United States west of the Rocky Mountains people who live near a volcano put their lives and prop- is considered to be a high-risk area for earthquakes, and erty at risk in the event of a volcanic eruption. Although all active volcanoes are in western states. Nevertheless, a volcanic eruption could occur regardless of whether or some parts of the central and eastern United States have not people live nearby, the fact that they do live in the been devastated by large earthquakes in the past, and vicinity leads to a risk and potential for disaster. in some ways are even more vulnerable to damage than A geologic hazard is a natural phenomenon, process, western areas (see Chapter 3). or event with the potential for negative effects on human life, property, or the environment. Examples include The Anthropocene and Planetary floods, tornados, hurricanes, volcanic eruptions, earth- quakes, and landslides. Poorly consolidated sediment on Boundaries a steep slope, for example, could pose a hazard to those As the human population and individual standards of living in a development on that slope. If slope failure, living have risen, our species has become the dominant known as a landslide, occurred during a heavy rainfall influence on many Earth systems. From increases in event, destroying homes and killing residents, it would greenhouse gases that warm the atmosphere to changes be a disaster. in biogeochemical cycles or wholesale reorganizations Natural disasters typically are sudden environmen- of landscapes and water supplies, our species is leaving tal changes that happen as a result of longer-term geo- its mark on the surface of planet Earth. Some Earth logic processes and have significant impacts on human scientists, in fact, have suggested that changes from life and property. The geologic hazard of earthquakes, human activities are so substantial and widespread that for example, is the result of hundreds or thousands of a new geologic era could be added to the geologic time years of accumulation of geologic deformation, but the scale, one called the Anthropocene.10 breaking point that causes an earthquake occurs rela- Decisions made now will determine whether future tively suddenly. generations inherit a degraded planet quite different Natural disasters can pollute the environment just from the one we live on presently, or a planet capable of as human-made disasters, such as oil spills and air pollu- providing for human and ecosystem needs over the long tion from factory smoke, do. Carbon dioxide gas seep- term. In 2009, sustainability scientist Johan Rockström ing from a volcano in Cameroon, Africa, for example, and his colleagues published a paper entitled “Planetary accumulated at the bottom of a lake in a crater atop the Boundaries: Exploring the Safe Operating Space for volcano and suddenly belched from the lake in 1981. Humanity.”11 In it, they identified Earth system pro- The geologic event suffocated and killed thousands of cesses critical to human well-being and assessed how sleeping villagers within minutes. those processes have been altered by human hands. Volcanic eruptions and earthquakes, geologic haz- They noted that the past 11,000 years, a warm period ards caused by the interior processes that move Earth’s called the Holocene epoch that postdates a major glacial tectonic plates, occur with little impact from human advance and cold period, has been a time of relative activities; an exception is wells that inject wastewater stability. During that time people developed agriculture underground, which have been documented to cause and irrigation, built permanent settlements that became relatively small earthquakes. Floods and landslides, in cities, and made phenomenal technological advances contrast, are geologic hazards that can be worsened that recently included exploring outer space and sending markedly by human activities. Removing natural vegeta- a remotely controlled rover to Mars. tion from hillslopes, for example, can increase the runoff The scientists cautioned, however, that this long- of rainwater and thereby increase flooding in streams. term stability might be transitory and that human Removing vegetation also destroys roots, which anchor activity might be capable of tipping Earth into entirely the soil on the slopes. Landslides occur when loosened new environmental states that could be disruptive to soil and other debris slips suddenly downhill. society. They sought to identify thresholds for critical CHAPTER 1 Dynamic Earth Systems 29 environmental processes that might lead to such tipping Rockström and his colleagues in the chapters of this points. Operating within a safe space for humanity, book. In carrying out our analysis we make use of the within the limits of what the scientists called planetary deep time perspective of Earth science, which provides boundaries, requires staying within those thresholds and us with myriad examples of former planetary states not reaching those tipping points. quite different from the present. In the last chapter of Rockström and his colleagues developed a list of this book, we revisit the ideas of planetary boundaries nine factors for which exponential growth of the human and humanity’s safe operating space. population and its associated consumption of resources and generation of wastes could exceed these limits and  As Earth entered a period of relative global threaten planetary stability (Figure 1-26). These nine warmth about 10,000 years ago, people began to boundaries range from local to global in their scale of develop agriculture (the Agricultural Revolution) process, and the threshold behavior of each ranges from and, since then, they have altered dramatically slow, with no evidence of planetary scale behavior, to the composition of ecosystems on Earth. The sharp and global in scale. The boundaries include the environmental ramifications of agriculture have been global cycling of major nutrients critical to life, climate, substantial. and other Earth systems (i.e., the biogeochemical cycles  Since the Agricultural Revolution, the rate of of nitrogen, phosphorus, and carbon); the global cycling human population growth has been rapid, accelerating of water; changes in land use; the loss of biodiversity in the years since the Industrial Revolution. Most of that is critical to self-regulating the biosphere’s resil- the recent growth is attributed to a decrease in death ience; aerosol loading to the atmosphere; and chemical rates because of better sanitation and medical care. pollution. While the absolute values of planetary thresholds  With industrialization has come a marked increase are uncertain, the idea that Earth systems are intercon- in consumption of energy, in particular the use of nected and that humans are having a marked impact nonrenewable sources of energy (fossil fuels). on these systems is clear. Along with other topics,  Decreasing environmental degradation would we address the planetary boundaries identified by require that population growth stabilize; that we

THRESHOLD BOUNDARY CHARACTER

Sharp, with potential Slow, with no known global-scale threshold global-scale threshold

Global Climate change

Ocean acidification

Stratospheric ozone depletion

Changes to global P and N cycles

Atmospheric aerosol loading

Freshwater use SCALE OF PROCESS Land use change

Loss of biodiversity Local Chemical pollution

Figure 1-26 Scientists have identified nine planetary from slow with no known global-scale threshold (right) to boundaries that range in scale of process from local to global possibly sharp with a global-scale threshold (left). (bottom to top), and in scale of threshold for potential change 30 PART I Introduction develop perpetual sources of energy, such as solar flooding, landslides, earthquakes, and volcanism. energy; and that we decrease the use of nonrenewable Mitigating natural disasters requires increased effort resources that require extensive mining and that from geoscientists—to understand natural phenomena produce pollutants. and their causes, and from all of us—to plan wisely  As human population increases, so does the risk when selecting places to live and work. associated with natural geologic hazards such as

Closing Thoughts We began by discussing the possible causes of rap- to determine cause and effect and to predict future idly shrinking glaciers atop the highest mountain outcomes. in Africa, Mount Kilimanjaro. This example shows The scale of human impact on Earth is relatively how Earth scientists investigate interconnected Earth small in comparison with that of geologic processes systems. Greenhouse gases emitted from activities such as volcanism, continental drift, waxing and wan- that include burning fossil fuels for energy lead to ing ice ages, or mountain building, yet humans are global warming that, in turn, can cause glacial ice by no means an insignificant force. The signatures masses to shrink and change the fluxes and stocks of human activities can be detected worldwide, from of water in Earth’s hydrosphere. We also examined extinct species to wholesale changes in land cover or the ways in which scientists investigate Earth pro- the composition of our atmosphere. Whether or not the cesses, using a multitude of tools and an overarching ongoing anthropogenic changes in the cycling of matter view of Earth as a dynamic system. By systematically and energy on Earth result in an environment in which examining each of the parts and processes within our we can live sustainably and within planetary boundary planet’s various subsystems, Earth scientists are able thresholds remains to be seen.

Summary

 Earth is best understood from an Earth system per- energy—solar energy, the energy from radioactive spective because the activities of our planet are inter- decay of minerals in Earth, and gravitational energy. related by the cycling of matter and energy through  Geologic processes, such as plate tectonic move- Earth’s various systems. ments or volcanism, can affect multiple Earth systems  The environment can be divided into six major over a period of time, each change resulting in another. systems: the geosphere (rock), pedosphere (weathered  Earth systems are characterized by both reinforcing rock and soil), hydrosphere (water), atmosphere (air), and balancing feedback processes when they respond to biosphere (life), and energy system. changes in conditions.  Cycling of matter and changes in the Earth sys- tem are the result of processes that are driven by

key terms

Earth system science biosphere (p. 5) fluxes (p. 6) dynamic system (p. 7) (p. 5) atmosphere (p. 5) open system (p. 6) static system (p. 7) geosphere (p. 5) sustainability (p. 6) closed systems (p. 6) energy (p. 7) pedosphere (p. 5) reservoir (p. 6) isolated systems steady state (p. 7) hydrosphere (p. 5) stock (p. 6) (p. 6) linear growth (p. 7) CHAPTER 1 Dynamic Earth Systems 31 exponential growth lower mantle (p. 11) kinetic energy (KE) pollution (p. 26) (p. 7) mantle mesosphere (p. 18) overpopulation reinforcing feedback (p. 11) potential energy (PE) (p. 26) (p. 8) outer core (p. 11) (p. 18) overconsumption balancing feedback inner core (p. 11) thermal energy (p. 19) (p. 26) (p. 8) rock cycle (p. 12) insolation (p. 20) pollution per unit of silicate minerals igneous rock (p. 12) resource (p. 24) resource used (p. 27) (p. 9) sedimentary rock (p. 12) potentially renewable geologic hazard (p. 28) core (p. 10) metamorphic rock resources (p. 24) natural disasters (p. 28) crust (p. 10) (p. 12) nonrenewable resources risk (p. 28) mantle (p. 10) soil (p. 13) (p. 25) planetary boundaries lithosphere (p. 11) critical zone (p. 13) perpetual resources (p. 29) asthenosphere (p. 11) hydrologic cycle (p. 15) (p. 25)

review questions

1. Explain how scientists do research aimed at 5. How are the lithosphere and crust different from determining the cause(s) of shrinking glaciers on each other? mountains such as Mount Kilimanjaro. 6. What are the major processes that operate in the 2. Why are Earth’s systems considered to be open? Why lithosphere and rock cycle? is the whole Earth system considered to be closed? 7. What are the major processes that operate in the 3. Is the amount of annual solar radiation (energy) hydrosphere? reaching Earth’s surface an example of a flux or a 8. What are the two most abundant elements in stock? Earth’s modern atmosphere? 4. How and when did the geosphere, hydrosphere, 9. What are the three main sources of energy on pedosphere, atmosphere, and biosphere form on Earth? Earth?

thought questions

1. What do you predict would happen if the flux runoff, and consequently even more soil erosion. of rock from Earth’s mantle were to increase? Is the preceding an example of a reinforcing or a For example, there is some evidence that periods balancing feedback? Explain your answer. of intensified volcanic activity have occurred 3. Why would scientists try to model Earth systems? during Earth’s history. How would this affect the What would be the benefits and limitations? rock cycle? 4. Why can’t plants (and hence food or trees) be 2. Development of an area that was tropical forest grown easily in the pedosphere if the upper meter or results in removal of all vegetation and exposure so of soil is eroded away? of bare soil. Rainfall runs off the devegetated surface and increases the flow of water in streams. 5. Can the oceans ever contain more water than they Increased runoff also has greater power to remove do at present, and hence result in a rise in sea level? soil, thus causing erosion. As the amount of soil Can they ever contain less water than at present, decreases, the amount of infiltration of rainfall and hence result in a drop in sea level? Has either of into the soil is lessened, resulting in even more these types of change occurred in Earth’s history? 32 PART I Introduction

suggested readings

Allaby, M. The Encyclopedia of Earth: A Complete Luhr, J. F., ed. Earth: The Definitive Visual Guide. Visual Guide. Berkeley: University of California New York: Dorling Kindersley, 2007. Press, 2008. McPhee, J. The Control of Nature. New York: Farrar, Berner, E. K., and R. A. Berner. Global Environment: Straus and Giroux, 1989. Water, Air, and Geochemical Cycles, 2nd ed. McPhee, J. Annals of the Former World. New York: Princeton: Princeton University Press, 2012. Farrar, Straus and Giroux, 2000. Langmuir, C. H., and W. S. Broecker. How to Build a Smil, V. Cycles of Life: Civilization and the Biosphere. Habitable Planet: The Story of Earth from the Big New York: W. H. Freeman, 1997. Bang to Humankind. Princeton: Princeton University Press, 2012. Smil, V. The Earth’s Biosphere: Evolution, Dynamics, and Change. Cambridge, MA: The MIT Press, 2003. Hazen, R. M. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet. New York: Zalasiewicz, J. The Planet in a Pebble: A Journey into Viking, 2009. Earth’s Deep History. New York: Oxford University Press, 2012. Houtman, A., S. Karr, and J. Interlandl. Scientific American Environmental Science for a Changing World. New York: W. H. Freeman, 2013.